The invention relates to a plasma protein concentrating and drying technology, in particular to a method for preparing pig plasma protein powder with low ash content. In the invention, waste, namely pig blood left after pigs are killed is used as a raw material, and novel plasma protein powder is prepared by centrifugal separation, ultrafiltration concentration and spray drying technologies. The plasma protein powder prepared by using the method has low ash content, low volatile basic nitrogen content, high immune globulin content, high digestion rate, good solubility, good palatability, highsafety, balance nutrients and remarkable effects for promoting weaned pigs to grow, and can be used as a supplement source of feed protein. Through the effective utilization on the pig blood, not only the waste is turned into a valuable thing and benefits are brought to the society, but also the environments can be protected and the current green development tendency is met.